Protection circuits are commonly used to protect electrical circuitry from exposure to transient surges of excessive voltages, known as overvoltage conditions. Overvoltage conditions occur when the voltage exceeds a particular circuit design capability. Overvoltage conditions may ultimately damage or destroy electrical components which is highly undesirable in any circuit design.
An environment in which overvoltage may occur is a battery bus system for an automotive vehicle. In certain conditions, the vehicle's battery bus system voltage will peak above its normal range, commonly referred to as “load dump”. Without adequate protection, load dump can permanently damage the vehicle's electronic modules. For example, load dump may occur when jump-starting a vehicle. More specifically, the voltage may increase to a voltage that is many times the normal battery voltage when the jumper cables are disconnected, thereby eliminating a path for the current which is produced by the generator/alternator, as the generator regulator is generally unable to instantaneously reduce that current. Another example of load dump is when a generator is running at full current to charge the battery and a battery connection becomes momentarily disconnected due to mechanical vibration. The result is an increased voltage similar to the jump start situation as described above. Voltages in excess of maximum operating voltages, such as those that occur during load dump, can be highly damaging to an unprotected circuit.
In an attempt to overcome this problem, a protection circuit is used to either block the voltage from reaching the circuitry, or regulate the supply voltage to a level that is acceptable for the particular circuit being protected. Typically, the protection circuit dissipates power in semiconductor devices, such as Zener diodes. However, this type of semiconductor device has a current that rises in proportion with the battery bus voltage, causing a very high power dissipation which raises the temperature of the semiconductor device. Typically a semiconductor device has a maximum junction temperature less than 200 degrees Celsius. Junction temperatures that exceed this maximum may result in damage to the device, or cause the semiconductor device to fail. Transient voltages exceeding a maximum voltage limit may require power to be dissipated that will threaten to exceed this maximum junction temperature, thereby causing permanent damage to not only the semiconductor devices, but to the circuitry the semiconductor devices are designed to protect.
For some overvoltage conditions, power dissipation in semiconductors is a viable solution. However, when the energy to be dissipated becomes large, the cost of this method increases as the size of semiconductor device or the number of devices necessary to accommodate the energy dissipation also increases.
Another method for power dissipation uses a switched resistor load in which a much higher temperature is tolerated. However, when the switch is opened, the battery bus system voltage is allowed to rise to a level that may be destructive. Furthermore, when the switch remains closed for a long period of time in order to ensure that the voltage has dropped to an appropriate level, excessive power dissipation occurs in the circuit, which may lead to potential damage.
There is a need for a method of dealing with overvoltage conditions without exceeding an allowed temperature for devices in the protection circuit in a cost effective manner.